1. Field of the Invention
The present invention relates to TE-TM mode converters, and more particularly, relates to a TE-TM mode converter using an electrooptic effect.
2. Description of the Related Art
As a related TE-TM mode converter, a polarization controller 100 described in Japanese Unexamined Patent Application Publication No. 2003-202532 has been proposed. FIG. 12 is a perspective view showing the appearance of the polarization controller 100.
The polarization controller 100 shown in FIG. 12 has a ferroelectric substrate 101, a waveguide 102, a mode conversion portion 103, and plate electrodes 104a and 104b. The mode conversion portion 103 includes domain regions D1 to D5 disposed along a light-wave traveling direction in the waveguide 102. In addition, the plate electrodes 104a and 104b are provided on the mode conversion portion 103 so as to sandwich the waveguide 102.
In the polarization controller 100, when a control voltage is applied to the plate electrodes 104a and 104b, an electric filed is generated parallel to a primary surface of the ferroelectric substrate 101 and perpendicular to a longitudinal direction of the waveguide 102. Accordingly, the principal axis rotation of the index ellipsoid of the waveguide 102 occurs, and two types of light waves (TE mode and TM mode) having planes of polarization, which are perpendicular to each other, are coupled, so that a TE-TM mode conversion occurs. In addition, since the domain regions D1 to D5 are disposed so that the polarization directions are alternately opposite to each other, they function as a grating which performs phase matching between the two types of light waves. As a result, the polarization controller 100 can perform the TE-TM mode conversion with high conversion efficiency.
However, the polarization controller 100 performing the TE-TM mode conversion by the grating has a problem in that the bandwidth of light waves, in which the TE-TM mode conversion can be performed, is narrow. In particular, as a communication wavelength for optical communication, a light wave having a wavelength of about 1,550 nm, which belongs to the C-band (1,530 to 1,565 nm), is used in many cases. Hence, in order to perform the TE-TM mode conversion for the above light wave, it is necessary to drive the polarization controller 100 in a bandwidth of 35 nm. However, it has been difficult to drive the polarization controller 100 in a bandwidth of 35 nm.